It seems to me that the omission of this option is/was based on the assumption that as groups go, only people to allowed to freely join a group should need to be able to see that group’s discussions.

I am sure that usually this assumption holds, but I can envision (and have encountered) a number of circumstances where this would not be the case, for example, support forums where only premium users can ask for help but where any user can read from existing threads.

Is there a plugin or hack for this that I just haven’t found yet?
Is the addition of this option in plans for upcoming updates?